1. Field of the Invention
This invention relates generally to a switch status monitor and specifically to a circuit which detects a welded motor controller relay contact in a garage door opener. The monitoring circuit provides a contact status signal to the garage door controller so that corrective action can be taken if the relay contact is welded closed.
2. Description of the Prior Art
In electrical circuits, it is often desirable to have a positive indication of the status of switch contacts to ensure that the contacts are in the proper position. This is particularly desirable in circuits employing relays which are often used to switch load current circuits. These high currents sometimes cause the relay contacts to stick together. The contacts are welded together by the electric arc occurring during switching. Other electrically operated switching devices such as triacs can also fail in a conducting state. If a switching device remains in a conducting state when it should not be conducting, damage to the load may occur or the continued, unwanted operation of the load may cause injury or damage.
In a system which opens and closes a garage door, relay contacts are often used to control a motor which moves the door. Welded contacts in a motor control circuit can cause the motor to continue operating when it should be stopped. For example, the door may continue closing after an operator has commanded the door to stop. Also, a safety system which detects obstructions to garage door closing could be rendered inoperative if a welded contact causes continued closing of the garage door after the safety system has detected an obstruction and commanded the motor to stop.
A number of schemes for monitoring contact status have been developed. Many use auxiliary contacts on the relay, however these do not permit direct monitoring of the main contact.
Other means use various forms of current detection. One such means disclosed in U.S. Pat. No. 4,914,315 to Nickolai compares the load signal to the relay coil signal. If they do not match, the circuit pulses the relay coil. Nickolai does not disclose the means of signal comparison.
Another means disclosed in U.S. Pat. No. 3,775,573 to Gaon sends a pulse through the contact which is detected if the contact is closed. U.S. Pat. No. 4,977,478 to Powell provides reference voltages on either side of the contact. Through resistors, these reference voltages provide a signal at a common junction which is compared to other reference voltages to determine contact status. These devices have the disadvantage that a reference signal must be supplied to the contacts and isolated from the signal which the contact is intended to switch. The switched signal must be of a kind which will not influence the detection circuit.
No device for monitoring the status of a garage door opener motor control contact is known to have been disclosed.